In the past, the HVOF (hypersonic velocity oxy-fuel) continuous spraying of higher melting point powdered materials such as tungsten carbide (in a cobalt matrix) has required the use of oxidizers of much higher oxygen content than that contained in air. for example, my earlier U.S. Pat. Nos. 4,416,421; 4,634,611; and 4,836,447 in particular, show forms of flame spray devices described as primarily oxy-fuel burners. Air may be one component of the oxidizer flow, but in each case the intensity of the flame jet relies on oxygen percentages greater than that contained in ordinary compressed air. The use of air to cool heated burner parts with this air subsequently entering and supporting the combustion process (regenerative cooling) was not feasible.
In place of "regenerative cooling", where the coolant becomes the oxidizing reactant, these prior flame spray devices rely on forced water cooling which severely limits the peak temperatures and jet velocities theoretically attainable. As an example, using a commercially available HVOF flame spray unit of the type discussed in U.S. Pat. No. 4,416,421, a simple heat balance shows that approximately 30% of heat released during the combustion process is carried away by the cooling water. Assuming a combustion peak flame temperature of 4,700 degrees Fahrenheit for a pure oxygen-propane mixture burning at a chamber pressure of 60 psig, if flame temperature was linearly related to heat content, then the 70% availability of the useful heat achieves a maximum flame temperature of only 3,150 degrees Fahrenheit. Of course, dissociation effects which limit the peak achievable temperature to 4,700 degrees F. release heat upon cooling. Thus, an actual combustion temperature of around 3,600 degrees F. is estimated.
Examining the combustion of compressed air and propane under conditions of essentially zero heat loss, the peak theoretical combustion temperature is about 3,400 degrees F. This is only 200 degrees F. less than that of the pure oxygen burner described above.
To now, in thermal spraying, it has become the practice to use the highest available temperature heat sources to spray metal powders to form a coating on a workpiece surface. It is believed that over 2,000 plasma spray units are in commercial use within the United States. These extreme temperature devices operate (with nitrogen) at over 12,000 degrees F. to spray materials which melt under 3,000 degrees F. Overheating is common with adverse alloying or excess oxidation processes occurring.
Recently, the HVOF (hypervelocity oxy-fuel) process has replaced many plasma applications for spraying heat-sensitive metals. Using pure oxygen as the oxidizer, flame temperatures of well over 4,000 degrees F. are realized. Thus, these devices also raise the powder particle to the melting point prior to impact against the workpiece surface. Adverse alloying mechanisms and oxidation still take place although at a lesser rate than for plasma torches.
In U.S. Pat. No. 5,129,582 for an HVAF (hypervelocity air-fuel) burner, it has been found that the quality of sprayed coatings of tungsten carbide powder with 13% cobalt is superior to HVOF-applied coatings of the same material. The improvement lies in the fact that the in-transit temperature of the powder particles is below the melting point. Additional heat to provide fusing of these particles is attributed to the conversion of kinetic energy to thermal upon impact against the workpiece surface.